Fig. 6 Relationship between residual NO3-N in the 0–100 cm soil layer (Nit0−100) and accumulated NO3-N in 100–300 cm soil layer (Nit100−300). (a) All accumulated NO3-N data. (b) Data eliminated when Nit100−300 > 110 kg ha−1. (c) Data eliminated when Nit100−300 < 110 kg ha−1. The horizontal red dashed line for 110 kg ka−1 is equal to Nit100−300 when the experiment was initiated. The vertical red dashed line for Nit0−100 of 101 kg ha−1 was calculated safe threshold for Nit0−100, which was defined as 100 kg ha−1 to facilitate calculations in this study. The vertical red dashed line for 158 kg ha−1 is the turning point where Nit100−300 increased sharply.